The spondyloarthropathies are a group of chronic rheumatic diseases of unknown etiology. They are strongly associated with the presence of the HLA Class 1 allele B27 and infection with enteric bacteria such as Salmonella, Shigella, Campylobacter, Yersinia, and Klebsiella. The mechanism by which the illnesses are related to B27 and enteric bacteria is not known. The applicant's work has shown that the hypervariable regions of B27 and proteins from enteric bacteria tend to share short sequence when compared to the other HLA-B alleles. Furthermore, contiguous with the B27 shared sequence, proteins from enteric organisms tend to satisfy the b27 binding motif. This convergence of both shared sequence with B27 and B27 binding motif occurs significantly more frequently in proteins from enteric bacteria than proteins of other origins. Finally, B27 contains a peptide in its third hypervariable region that fulfills the B27 binding motif. Experimental evidence demonstrates that this B27 derived peptide can be bound by B27 as are several peptides derived from enteric bacterial proteins. Based on these findings, a new hypothesis for the relationship of spondyloarthropathies to B27 and enteric bacteria will be tested. This hypothesis is based on convergent evolution between B27 and proteins from enteric organisms, the structure of B27 which includes a peptide bound by B27, and peptides from proteins of enteric bacteria bound by B27. In Aim 1, peptides that share pentapeptide or hexapeptide sequence with B27 and that also fulfill the B27 binding motif will be studied for their ability to bind B27 in a solution phase assay utilizing the recombinant, extracellular portion of B27, beta-2-microglobulin, and peptide. In Aim 2, as data accumulates on peptide binding to B27, the motif will be revised. Other database searches will be performed to identify new peptides from enteric bacteria that share shorter or degenerate sequences with B27 and fulfill the revised motif. These peptides will then be tested for their ability to be bound by B27. In Aim 3, peptides that are bound by B27 will be evaluated for their ability to be recognized by T lymphocytes from patients with spondyloarthropathies, normal controls or patients with other rheumatic illnesses. In Aim 4, studies will be undertaken to determine if the peptides identified in Aims 1 - 3 are presented by MHC Class I in B27-positive, intact cells. Finally, in Specific Aim 5, the peptides identified in the above experiments will be studied for their ability to modulate clinical illness, utilizing the new animal models of spondyloarthropathy that are present in animals transgenic for B27.